The present invention relates to an optical isolator using a Faraday rotator. Optical isolators have been applied to optical communications such as in systems using optical fibers to unidirectionally transmit light. This type of optical isolator prevents light reflected by an optical component such as a connector or an optical switch from being returned to a laser source, thereby stabilizing laser oscillation.
FIG. 1 shows an arrangement of a conventional optical isolator.
Referring to FIG. 1, light I coming along the direction indicated by an arrow enters a polarizer 1 and then is linearly polarized. The linearly polarized light from the polarizer 1 is transmitted to a Faraday rotator 3 made of paramagnetic glass placed in a magnetic field produced by magnets 2. The polarization direction of the light is rotated by the Faraday rotator 3 and the rotated and polarized light is transmitted to a polarizer 4. Since the polarization axis of the polarizer 4 substantially coincides with the polarization direction of light emitted from the Faraday rotator 3, the light from the Faraday rotator 3 is transmitted through the polarizer 4 However, when light reaching the Faraday rotator 3 from the side of the polarizer 4 is transmitted to the polarizer 1 through the Faraday rotator 3, it is blocked since the polarization direction of the light emitted from the Faraday rotator 3 does not coincide with the polarization axis of the polarizer 1 (i.e., in general, the polarization direction is perpendicular to the polarization axis). Reflection and antireflection films are coated on the end faces of the Faraday rotator 3, and the incident light is reflected in a zig-zag manner so as to obtain a large Faraday rotation angle.
Cubic polarizing beam splitters or Glan-Taylor polarizing prisms are used to constitute the polarizers 1 and 4. The cubic polarizing beam splitter comprises two isosceles triangular prisms having identical shape and size. The main sectional surface of each prism perpendicular to a corresponding lateral edge has an isosceles triangular shape. The base surfaces of these two isosceles triangular prisms are bonded through a polarizing film. The input and output end faces of the cubic polarizing beam splitter or the Glan-Taylor polarizing prism are perpendicular to the optical path of the incident light. For this reason, light reflected by the input and output end faces returns along the same path as the incident optical path, thereby impairing performance of the optical isolator for preventing return of light.
In order to solve the above problem, each polarizer can be inclined with respect to the optical path of the incident light to prevent the reflected light from being returned along the same optical path as that of the incident light. However, when the polarizer is inclined by an angle of 5.degree. or more with respect to the optical path of the incident light, the angle of incidence of light on the bonding surface of the prisms is inevitably deviated from an optimal value (45.degree.). As the result a ratio Tp/Ts (i.e., an extinction ratio of a transmittance of a p wave to that of an s wave is lowered. More specifically, as shown in FIG. 2, the extinction ratios at an incident angle .theta.1 of the incident light at the end face of the polarizer and at an incident angle .theta.3 of the incident light to the bonding surface of the prisms are greatly lowered when the incident angles .theta.1 and .theta.3 are more than 7.5.degree. (less than -7.5.degree.) and less than 40.degree. (more than 50.degree.), respectively.
Even if an inclination angle of the polarizer is small, when a distance between a light source and the polarizer is increased, it is possible to prevent the reflected light from being returned to the light source. However, the size of the overall optical system is increased. In addition, although it is possible to coat antireflection films on the input and output end faces, reflection cannot be completely eliminated.